In the field of graphic art, a set of separated color images are prepared from a color original sheet using a lith type film, and a final color image sheet is prepared using the separated color images. Prior to the final printing, a color proof is generally prepared for checking any mistakes possibly introduced in the preparation of the set of separated color images and further checking whether color adjustment is required or not. A paper sheet is generally employed as the material for preparing the color proof because the color proof should be as analogous as the finally printed paper sheet. For the same reason, a pigment is preferably employed as coloring material. Further desired is a high resolution so that a half tone is precisely reproduced. Furthermore desired is an enhanced reliability of the process.
Recently, there arises a demand for a process for preparing a color proof by a dry process, namely, a development process using no developing solution.
At the present time, the stage prior to the printing, namely, a prepress, is highly computerized. Therefore, a process and material for directly reproducing a color proof from a set of digital signals is required. In such computalized system, it is needed to produce a color proof of extremely high quality. Generally, an image of at least 150 lines/inch is required. For preparing a proof of such high quality from digital signals, a laser light which is highly coherent and can be modulated by digital signals should be employed as a recording head. Therefore, it is required to develop a recording material which shows high sensitivity to a laser light and shows such high resolution as to reproduce a very fine dots.
Japanese Patent Provisional Publication (for PCT application) No. 2-501552 discloses a recording material which is employ-able for reproducing an image of very fine halftone by means of a laser light. The recording material comprises a transparent support, an image forming surface layer which turns fluidal upon receiving a heat, and an image forming material layer of porous or granular material. When a laser light is applied, the image forming material layer in the area exposed to the laser light is fixed onto the support. Then, the unexposed area of the image forming material layer is peeled off to leave an image formed of the exposed image forming material layer on he support.
In the above image forming method, the image is formed directly on the transparent support. Therefore, the employable support is limited. Further, it is not easy to prepare of an image of multi-color. Accordingly, this process is not appropriate for employment as a method for preparing a color proof which generally needs the use of a paper sheet (i.e., pulp paper sheet) and on which a multi-color image is generally formed.
Japanese Patent Provisional Publication No. 6-219052 describes an image transfer sheet which comprises a support, a light-heat conversion layer of a light-heat conversion material, a thermally activable releasing layer of very small thickness (such as 0.03 to 0.3 .mu.m), and an image forming layer comprising a coloring material. In this image transfer sheet, the bonding force between the image forming layer and the light-heat conversion layer decreases in the area where a layer light is applied. Such decrease of the bonding force is caused by thermal deterioration of the releasing layer. If an image receiving sheet is beforehand provided on the image forming layer when the laser light is applied to the image transfer sheet, an image of the area exposed to the laser light is transferred onto the image receiving sheet. In this system, the transfer of image is accomplished by so called "ablation". In more detail, the releasing layer decomposes to produce a gas in the area exposed to the laser light, and hence the bonding strength between the light-heat conversion layer and the image forming layer decreases in that area. The image forming layer on that area is then transferred onto the image receiving sheet. The image forming system utilizing the "ablation" is favorable in that a paper sheet having an adhesive undercoat can be employed as the image receiving sheet and a multi-colored image with fine tone can be easily prepared by placing the image transfer sheets of different colors on the image receiving sheet by turns. Accordingly, this method is advantageously employable for preparing a color proof (particularly, DDCP: Direct Digital Color Proof) or an extremely fine mask image.
The above-mentioned color forming system of Japanese Patent Provisional Publication No. 6-219052 was invented by inventors including one of the inventors of the present inventors.
As a result of further study on the above color forming system, it has been noted that while the image formation using the image transfer sheet having the releasing layer in combination with an image receiving sheet provided on the transfer sheet gives a highly fine image, the obtained image sometimes is apt to be fogged. Such fogging is observed specifically when the laminate composed of the image transfer sheet and the image receiving sheet is kept for a certain period of time without separating the receiving sheet from the transfer sheet after it is exposed to the laser light. The fogging appears to be caused by transfer of the image forming layer in the unexposed area. Such fogging is unwelcome, particularly in the preparation of a color proof of high quality.